Among the several types of flow measurers, the flow measurer based on differential pressure is one of the most well-known. One of the elements that makes part of this kind of measurer is a device through which a pressure differential is produced (Venturini; hole board, nozzle); the other important element is the pressure transducer, that is liable for recording the pressure variation (charge loss) produced when the fluid flows through a Venturini.
The patents' literature presents several documents on this subject. Patent number U.S. Pat. No. 4,899,046 instructs a sensor in which a sealed cavity is formed of two half cavities (20,22) with flanges that allow they are united. The two half cavities provide form to the chambers (26,28), that are separated by a diaphragm (24). The area surrounding the diaphragm is sealed by the connection of the same two half cavities. Full of pressure (30, 32), they allow the communication of the liquid with the chambers, for each chamber, whose pressure difference must be measured. Na optical guide (34) is attached to the diaphregm so that the pressure differences are transmitted to it. A source sents a light (40) to an optical-electronic converser (42) attached to a signal processor circuit (44) and to an indicator (45). In the proportion the diaphragm deforms itself under pressure, the light through the guide shall become low.
The International publication WO9944026 (U.S. Pat. No. 6,563,970) instructs Bragg Network for sensoring the temperature and pressure in oil wells. The novelty in this invention is that the supports (5a, 5b) form a pressurized body (7a, 7b) which converts the surrounding pressure of a medium (11) into expansion or longitudinal compression of the pressure sensor fiber (3). The sensor (1) comprises an isolated a pressure-isolated chamber with the entrance of a fiber (16) sealed by its own pressure force. A fiber detects the pressure and the temperature. The fiber of the temperature sector (19) includes a FBG (20). The pressurized body comprises a pressure cylinder (7a, 7b), whose cavity (9) presents an opening (118), and the end surfaces (8a, 8b) are connected to the fiber holders (6a, 6b). The first support (5a) is a (5a) cylinder (7a) that is expandable due to the internal pressure; the second one is an external cylinder (7b) isolated from the surroundings (11); both are attached to the transducer's wall (14) and are encapsulated by its packing. Alternatively, in a similar assembly, the external cylinder is compressed by the external pressure. It is described additional variations based on similar principles. Several transducers with different Bragg wave lengths are attached by optics fiber for a large light source range (40) to a detector unit. This comprises a multiplexer and a wave length detector connected to the electronic instrumentation. The pressurized body is full of silicon oil and the pressure-isolated chamber has low pressure or vacuum gas. According to its inventors, this sensor could be used to measure the pressure and the temperature in oil wells above 100 MPa (14500 psi) and up to 170° C.
In the sensor described in the International Publication WO 0114843-A1, it is employed at least two Bragg networks. In this pressure sensor, the use of two networks allows the set off of temperature variations and possible effects over the wave length of the FBGs due to the contact with gases, possibly the H2. The pressure transference element converts the pressure of a medium into na expansion or longitudinal compression of at least one section of fiber optics with one Bragg network. The sensor which is the object of the invention is used to measure high pressures, i.e., within the 100 Moa (14500 psi), especially in natural gas or oil wells.
The International Publication WO 0033048-A2 proposes a pressure sensor based on fiber optics using Bragg networks, also aiming at well applications. It is used one fiber optics with two FBGs. The fiber section which contains one of the Bragg network is fixed with an epoxi-based glue to a pressure detector. The elastic deformation of the detector is transmitted to the FBG along its longitudinal central line, so that mechanical restrictions corresponding to the deformation are developed in the FBG. The wave length which characterizes FBG's reflectiveness is modified according to the mechanical restrictions. Another FBG is put in the pressure detector to Record the environmental temperature. According to the writers, this sensor is suitable for high pressures and it may be used to measure the pressure in wells engines, combustion chambers and other environments.
The U.S. Pat. No. 6,597,821 patent defines a differential pressure sensor that uses a fiber laser and comprises a light source, supply fibers, a fiber laser formed by at least two reflectors at their ends, one detector and one analyser. The measuring principle is based on the induced pulsation by birefringence. The invention's application indicated by its inventors its the pressure measurement in oil fields.
The scientific literature reports an article written by Lim, J., Yang, Q. P., Jones, B. E., and Jackson, P. R, “DP Flow using Optical Fiber Bragg Grating,” Sensors and Actuators A, vol. 92, pp 102-108, 2001, in which it is reported the development of an optical sensor with flow based on the differential pressure. The differential pressure sensor uses two Bragg networks in simple optics fiber that are mounted in both sides of the differential sensor diaphragm. The flow signalling is got by the modification of the wave lengths of the two FBGs. The sensor was tested with flow rates of 800 cm3/s, with the temperature varying between 25° C. and 75° C., with maximum pressure differential of 0.08 Mpa (12 psi) produced by a hole board. It has been also tested with a monitoring system for the hydraulic valve, with fluid's rate above 6 cm3/s, and at the pressure of 0.7 MPa (100 psi). The main difference between the differential transducer and the sensor proposed in this invention is that, contrary to the sensor reported by Lim et al, that employed FBGs in series with the support of na optical coupler, in the Requiring Party's sensor, the Bragg networks fixed in opposite sides of the diaphragm are positioned in series in the same optical fiber. This in series arrangement makes the integration easier (and reduces the losses by insertion) with the other sensors to the Bragg network of the intelligent optical completion system developed by the Requiring Party.
Thus, despite the development of the techniques, there is still the need of a Fiber Optics Differential Pressure Transducer in which the differential pressure produced by a Venturini type device acts directly over a diaphragm surface, that is inflected, being the deformation produced by the pressure action in the diaphragm surface related in line with the pressure applied and transformed into optical deformation using na FBG mounted on the center of the diaphragm in the side that is under stress, while a second FBG mounted on the diaphragm surface that is under compression is used to measure the temperature, providing a set off for the temperature, being such differential pressure transducer described and claimed in this application.